This invention relates to a data communication system used for transmission and reception of periodic data between a plurality of computers and relates also to such a data communication method.
A cyclic communication system disclosed in, for example, JP-A-4-133540 is known as a data communication system of this kind.
The cyclic communication system disclosed in the application cited above is a communication system in which a plurality of control computers are coupled to a transmission channel (a data transmission channel) for transmission and reception of cyclic data between these plural control computers. Each of these control computers includes a plurality of CPU's, a shared memory, a system bus, a communication controller and a cyclic memory. In each of the control computers, the CPU's, the shared memory and the communication controller are coupled to each other by the system bus respectively, and the communication controller coupled to the cyclic memory is connected to the data transmission channel. Among cyclic data stored in the cyclic memory, those cyclic data having a higher access frequency than the others are selected, and the selected cyclic data are transferred to and stored in the shared memory, while the other cyclic data are stored in the cyclic memory only. Because of the above arrangement, each of the CPU's is only required to make access to the selected cyclic data stored in the shared memory through the system bus when access to the selected cyclic data is necessary, and there is no need for activating the communication controller.